This invention relates to a structure for the transport, installation and dismantling of an offshore oil rig deck.
This invention relates to a process for the transport, installation and dismantling of an offshore oil rig deck.
In the extraction of oil it is known that an oil rig which comprises a deck bearing in particular operating equipment and accommodation is positioned above an oilfield.
The deck is supported by a supporting assembly comprising legs which bear against the sea bed or floating columns anchored to the sea bed.
Several methods are used to transport, install and dismantle oil rig decks.
One known method comprises using lifting cranes mounted on barges to transfer the rig deck from the transport barge to the supporting assembly. This method, which is the one in most widespread use at the present time, has limitations. The first of these limitations is the lifting capacity of the lifting cranes, which may make it necessary to construct the deck in several sections, significantly increasing the manufacturing cost of the deck and the cost of installing and dismantling the said deck on the oil rig. The second limitation lies in the fact that this method makes it necessary to have a relatively large favourable weather window in order to be able to carry out this operation at sea under satisfactory conditions.
Another known method comprises installing the oil rig deck as a single block on the supporting assembly by causing it to float. The deck is then placed over the supporting assembly by means of either a ballasting system or a mechanical system.
The main disadvantage of ballasting or deballasting systems lies in the fact that they depend on sea conditions, which makes it difficult to use them in areas where the favourable weather windows are relatively short.
In general the mechanical systems used hitherto to install and dismantle oil rig decks are faster than ballasting or deballasting systems.
Furthermore, a structure for the transport, installation and dismantling of a fixed oil rig deck which comprises a U-shaped floating hull fitted with at least three legs for lifting that hull and designed to bear against the sea bed and a shuttle which can move along the said legs on the hull and intended to be applied to the underside of an oil rig deck is known from FR-A-2 837 461.
Thus when the rig deck is installed or dismantled the legs are lowered to bear against the sea bed and the deck is lifted by the shuttle, which is itself lifted by the hull.
In the case of oilfields in deep water this type of structure cannot be used for the installation or dismantling of an oil rig deck because the legs have to be in contact with the bottom in order to lift the deck, which is impossible in the case of such fields.